This invention relates to the reduction of an imaging time and an improvement in image quality in a magnetic resonance imaging apparatus for imaging a desired portion of a subject by utilizing magnetic resonance.
A magnetic resonance imaging apparatus (hereinafter referred to as the "MRI" apparatus) measures a density distribution of nuclear spins, a relaxation time distribution, etc., at a desired portion to be inspected inside a subject by utilizing nuclear magnetic resonance, and displays the image of a section of the subject from the measurement data.
A nuclear spin of the subject placed inside a homogeneous and strong static magnetic field generator causes precession in a direction of a static magnetic field as an axis with a frequency (Larmor frequency) which is determined by the intensity of the static magnetic field. When a radio frequency pulse having a frequency equal to this Larmor frequency is irradiated from outside the subject, the nuclear spin is excited and shifts to a high energy state (nuclear magnetic resonance). When the irradiation of the radio frequency pulse is cut off, each nuclear spin returns to its original low energy state with a time constant corresponding to the respective state, and emits an electromagnetic wave (NMR signal) to the outside. This signal is detected by a radio frequency reception coil tuned to its frequency. To add position data to the detected signal, the MRI apparatus applies gradient magnetic fields of three axes corresponding to a slice direction, an encoding direction and a readout direction to the space of the static magnetic field. As a result, the signal from each position inside the subject can be separated and distinguished as frequency data.
Next, an image reconstruction method in the MRI apparatus will be explained. To begin with, a pulse sequence in a spin echo method employed generally will be explained. FIG. 2 of the accompanying drawings shows the pulse sequence. There are two kinds of radio frequency pulses (RF pulses), that is, a 90.degree. pulse and a 180.degree. pulse. These pulses are applied to the subject with the slice gradient magnetic field to excite the nuclear spins in the section or slice to be imaged. When the slice gradient magnetic field G.sub.z is applied in a Z-axis direction and the RF pulses are applied as shown in FIG. 1, the nuclear spins of only the frequency portion of the RF pulses on the Z axis are excited. The 90.degree. pulse is first applied and immediately thereafter the readout gradient magnetic field Gx is applied to promote phase diffusion of the excited spins. Next, when the 180.degree. pulse is applied, the nuclear spins are excited by 180.degree., and the diffusing direction of the nuclear spins reverses. When the readout gradient magnetic field is again applied, the nuclear spins converge and generate sharp echo signals in the time twice the pulse interval between the 90.degree. and 180.degree. pulses. This time is referred to as the "echo time" (Te).
The echo signal hereby obtained represents one dimensional projected image data in the readout direction (X-axis direction in FIG. 1), but a two-dimensional image cannot be reconstructed by this data alone. Therefore, the gradient magnetic field Gy is applied in an encoding direction (Y-axis direction) as still another axis during the application of the readout gradient magnetic field which provides the phase diffusion between the 90.degree. and 180.degree. pulses in order to impart phase rotation to the nuclear spins in accordance with positions in the Y-axis direction. In this way, data in the encoding direction is superposed on the echo signal as phase data. Furthermore, the echo signals are measured repeatedly by applying this encoding gradient magnetic field while changing its intensity. In this case, the time between the 90.degree. pulses is referred to as the "repetition time" (Tr). Positive and negative numbers are imparted to the encoding quantity. When the encoding number is zero, the quantity of the encoding gradient magnetic field is zero, and positive and negative encoding quantities are imparted.
Generally, a number of times of encoding is from 128 to 256 to secure sufficient resolution of the image.
When the echo signal train obtained in this way is analyzed by two-dimensional Fourier transform means, two-dimensional image data can be obtained. An image reconstruction method by this two-dimensional Fourier transform will be hereinafter referred to as the "2DFT".
Next, image weighting which is clinically important for MRI images will be explained. The spins of protons include two kinds of relaxation which change in accordance with their existing environment, that is, a longitudinal relaxation and a transverse relaxation. A signal intensity S can be calculated from the relaxations as represented by the following equation (1): EQU S=.rho.(1-exp(-Tr/T1)).multidot.(exp(-Te/T2)) (1)
where .rho. is the density of the existing protons, T1 is the time required for the signal density to recover to 1-1/e when a saturation level of the signal intensity is "1" in the longitudinal relaxation and is a value inherent to a respective texture, and T2 is the time required for the signal intensity to recover to 1/e in the transverse relaxation and is a value inherent to a respective texture.
The second term on the right side of the equation (1) represents a recovery process of the signal intensity with respect to the repetition time Tr, and this is the longitudinal relaxation (which depends on the T1 value). The third term represents a decay process of the signal intensity with respect to the echo measurement time Te, and results from the transverse relaxation (which depends on the T2 value).
FIGS. 3A to 3E are explanatory views useful for explaining the relation between the measured echo signal intensity and Tr as well as Te. It will be hereby assumed that a subject has a texture A on the outer side and a texture B on the inner side. FIG. 3A shows the recovery process of the signal due to longitudinal relaxation, FIG. 3B shows the decay process of the signal due to transverse relaxation, and FIG. 3C show image contrasts when Tr and Te are combined in various ways.
When a T1 weighted image reflecting the T1 value is to be imaged, any influences of exp(-Te/T2) in the equation (1) are minimized by the use of a Te value which is as small as possible (represented by symbol S in FIG. 3A) so as to suppress the influences of the transverse relaxation, and a relatively small value is used for Tr (represented by symbol "S" in FIG. 3A; generally not greater than 500 ms in the human body) so that a T1 difference between the textures A and B can be extracted. In this example, the T1 value of the texture B is smaller than that of the texture A as shown in FIG. 3A. Therefore, signal recovery by the longitudinal relaxation is quicker in the texture B than in the texture A, and a signal having a higher signal intensity can be obtained in the texture B than in the texture A. As a result, a brighter image can be obtained in the texture B than in the texture A as shown in A of FIG. 3C. When a T2 weighted image is to be imaged, a signal of each texture is recovered by the use of a sufficiently long Tr (represented by symbol "L" in FIG. 3A; generally not shorter than 1,500 ms), and imaging is carried out while Te is set to a longer value (represented by symbol "L" in FIG. 3B; generally about 100 ms). In this example, since the T2 value of the texture A is longer as shown in FIG. 3B, an image having a reverse contrast relative to that of the T1 weighted image can be obtained as shown in B of FIG. 3C. When Tr is set to a long value with Te being set to a short value, an image which is not affected by the T1 and T2 values of each texture can be obtained as shown in C of FIG. 3C. This image is one relying on the proton density of the texture, and is referred to as a "proton density image". The T1 and T2 values change, even in the same texture, depending on the state (in the texture e.g. a tumor), and are therefore used to specify a disease portion.
The description given above explains the outline of MRI. For further details, refer to "NMR Medical Science (Basis and Clinical Application)", edited by the NMR Medical Science Society, Research Meeting, published by K. K. Maruzen, Jan. 20, 1984.
To obtain various weighted images useful for clinical study, the combination of Tr and Te optimal for a pulse sequence must be set in accordance with the texture to be imaged. However, to obtain the T2 weighted image, Tr must be set to a long value. Since the imaging time is given by the product of Tr and the number of times of encoding, there remains the problem that imaging cannot be finished within a short time. On the other hand, Te must be shortened to obtain the T1 weighted image. To satisfy this requirement, a reception band of signal measurement must be expanded. Therefore, there remains the problem that a signal-to-noise ratio (S/N) increases.